1. Field of the Invention
The present invention relates to an image forming apparatus and a developing device.
2. Description of the Related Art
Tandem-type image forming apparatuses are known in which a latent image is formed on each of a plurality of image carriers, each of the latent images is then developed into unicolor toner image by using a unicolor toner, the unicolor toner images are then sequentially transferred onto a transfer belt in a superimposing manner to obtain a full color image, and the full color image is then printed on a sheet of paper. Residual toner, which is toner that remains on the image carriers after the unicolor toner images are transferred onto the transfer belt, is cleaned by using a cleaner member such as a cleaning blade.
Cleaner-less type image forming apparatuses are known, in which residual toner is temporary captured, returned to the surface of the image carrier after a printing has been completed or at a predetermined timing, and collected by a developing unit. Conventional technologies are disclosed in, for example, Japanese Patent No. 3728166 and Japanese Patent No. 3597254.
However, when a paper sheet with a first unicolor unfixed toner image, say yellow image, thereon reaches to a subsequent image carrier for a subsequent second unicolor unfixed toner image to be transferred thereon, say magenta image, toner of the first unicolor unfixed toner image may be disadvantageously reversely transferred onto the subsequent image carrier. In the cleaner-less tandem-type image forming apparatus, reversely transferred toner of the first unicolor, i.e., yellow, is disadvantageously collected into a developing unit of the second unicolor, i.e., magenta. In the beginning, the proportion of the reversely-transferred yellow toner in the developing unit for magenta is low. However, as the magenta toner is consumed and more and more yellow toner accumulates in the developing unit as time passes, the proportion of the yellow toner in the magenta developing unit increases. As a result, a clear magenta image can not be formed. In other words, color reproducibility degrades over time.